Chain Folding and Unfolding Strucutres of Semicrystalline Polymers Elucidated by Solid-State NMR Spectroscopy

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NON-TECHNICAL SUMMARY:Many commercial plastics are made of "semicrystalline polymers", i.e. long-chain molecules that have the ability to crystallize when processed industrially. They play important roles in our daily lives due to excellent mechanical and thermal properties, lightness, and easy low-cost fabrication. Thus, various semicrystalline polymers are used in a wide variety of applications including packaging, textiles, automobile, airplane parts, energy, biocompatible materials, and so on. To further improve properties of semicrystalline polymers, understanding of their detailed structure in the solid state is necessary, but, despite many efforts over the last half century, the three-dimensional molecular structures of the polymer chains are still not well understood. This project focuses on fundamental understanding of molecular-level structures of semicrystalline polymers in samples crystallized and processed in various ways, and may lead to predictions of improved properties for commercially available polymeric materials. The PI will be using novel specialized spectroscopic techniques to probe and understand the shapes of individual polymer molecules in the solid-state samples. This project will also include advanced scientific training of graduate and undergraduate students. They will experience a wide range of polymer research covering polymer synthesis, crystallization, deformation, advanced characterizations, and simulations. The research results will be reported in scientific journals and presented by the researcher and students at national scientific meetings.TECHNICAL SUMMARY:Polymer crystallization involves conformational changes from random coils in the melt and solution states, to folded chains in the bulk and in single crystals with a typical thickness of ca. 5-20 nm. The crystallization of polymers driven by kinetics yields unique features in terms of crystal habits, lamellae thickness, and crystallinity, among other features. Various characterization tools have been developed to reveal hierarchical crystalline structures. Nevertheless, the understanding of the molecular-level conformations remains challenging due to limitations of experimental resolutions. The PI will employ a novel approach of 13C-13C double quantum (DQ) NMR combined with selective isotopic 13C labeling to elucidate re-entrance sites of folded chains, successive folding numbers, and adjacent re-entry fractions of semicrystalline polymers, including polyethylene, isotactic polypropylene, isotactic poly(1-butene), and poly(lactic acid) in the bulk and in single crystals as a function of undercooling from the melt. This work is aimed at providing detailed molecular pictures of semicrystalline polymers during crystallization as well as under deformation, in relation also to computer simulation results, and will enhance our theoretical understanding of the crystallization of polymeric materials.